Various types of replaceable circular fluorescent lamp units have been devised for use in incandescent sockets. In preheat operating circuits used in units of this type, when the glow bottle switch is closed the circuit is completed and the heating current flows through the cathode at each end of the lamp. After a short preheat time (usually about one second), the switch is opened. This impresses a high voltage pulse across the lamp and causes an arc to strike between the cathodes. Preheat lamps of the type discussed here usually are operated on a single lamp ballast and have a simple choke coil or a choke with an auto-transformer to supply the voltage required to start and operate the lamp.
When the fluorescent lamp in such a unit reaches the end of its life, repeated attempts to start it will fail. With either a thermal-switch starter or a glow switch starter, the cathodes will continue to flash on and off until the starter fails or the lamp is replaced. Furthermore, the ballast may be permanently damaged by overheating. To alleviate the problem of ballast overheating, one known type of this lamp unit has had a thermal breaker, or a one-shot thermal fuse, coupled to the ballast contained within the central housing of the lamp holder. Such a lamp holder normally works in conjunction with a package having a glow bottle switch and a capacitor integral with a replaceable circular lamp. One disadvantage of this type of lamp assembly, however, is the additional cost of replacing the circular lamp package containing the glow bottle and capacitor when the lamp has reached its end of life. This is necessary even though the glow bottle switch and capacitor may still be operative. Also, in the case where a thermal fuse is used, the entire lamp holding unit is rendered inoperable until such a fuse is replaced.
In many fluorescent lamp assemblies, the function of preventing ballast overheating has previously been performed by cut out protector (COP) starters of the manual reset or automatic reset types. This type of starter is housed in its own enclosure and is designed to be a user replaceable element in the operating circuit. The disadvantages of using these types of starters in many present circular fluorescent lamp assemblies include relatively high cost and, in the case of a normal reset type, the nuisance factor of accessibility to reactivate. In the case of the automatic reset type, disadvantages include the drawing of one watt of unused power for the heater contained within and the lack of protection against fixture misapplication or ballast failure while the lamp is in operation.
It is believed, therefore, that a lamp assembly which overcomes the several disadvantages associated with the prior art devices mentioned above would constitute a significant advancement in the art.